Boron phosphides (BP and B12P2) are desirable due to superior thermal stability (up to 1400 K in air), excellent chemical stability and high hardness, suitable for use as a promising abrasive material, which may be utilized for a wide range of engineering applications. The main limitation for the wide use of BP and B12P2 is the lack of relatively simple and economical methods for their production.
Polycrystalline boron phosphide powders were traditionally synthesized by direct reaction of the elemental boron and phosphorus in sealed silica tubes under a few atmospheres phosphorus pressure and heating (1400 K at least) for a prolonged period of several hours. Also, BP powder has been prepared by solvothermal co-reduction of a boron tribromide-phosphorus trichloride using metallic sodium as a reductant. Boron phosphide single crystals have also been grown by crystallization from metal solutions, or by chemical vapour transport reaction with sulphur. The main disadvantages of these methods are: the use of toxic and aggressive reagents, rather complicated technical implementation, high labour intensity and time consumption.
Recently, a self-propagating high-temperature synthesis of boron phosphides according to the following reactions has been developed (see Mukhanov et al., Journal of Superhard Materials 2013, 35, 6, 415-417 for BP, Mukhanov et al., Journal of Superhard Materials 2014, 36, 1, 18-22 for B12P2):BPO4+4Mg→BP+4MgO  (1)2BPO4+5MgB2+3Mg→B12P2+8MgO  (2)
These methods provide a simple, convenient and low-cost access to BP and B12P2, starting from a widely available boron containing reagent (boron phosphate BPO4 is the third boron containing substance (after sodium borate Na2B4O7 and boric acid H3BO3), and is commercially available (CAS Number: 13308-51-5).
However, this self-propagating high-temperature synthesis requires very high initiation temperatures (around 1000 K). Of note, even though the mixed reagents may be compressed into pellets or ingots prior to heating, such as described for instance in EP 2886515, the chemical reaction actually occurs only during heating. The mechanical energy provided by compression to the system is insufficient to initiate the reaction. In these documents, compression is only a way to ensure a good contact between the different reactants Indeed, in a self-propagating high-temperature synthesis, heating at high temperature (typically 1000K) is an essential feature for initiating the chemical reaction, and comes with side-reactions in particular through partial oxidation by air oxygen, resulting in a relatively low yield of the desired product (around 35% yield for BP, 50-75% for B12P2). Moreover, the process does not allow controlling of the particle size of the obtained boron phosphide powder.
Indeed, boron phosphide powders with a particle size of less than 60 nm are of particular interest as it is envisioned that such powders would lead, after sintering, to materials which could compete with diamond as super-hard materials.
There is therefore a need for a simple, convenient, low-cost and tunable process for the preparation of boron phosphides, which would in particular allow production of BP and/or B12P2 powders with a controlled size of the boron phosphides particles thus obtained, depending on the process parameters.